U.S. patent application number 14/377109 was filed with the patent office on 2015-01-01 for technique for designing and manufacturing solid oxide fuel cell having improved output capability in mid to low temperature.
This patent application is currently assigned to KOREA INSTITUTE OF INDUSTRIAL TECHNOLOGY. The applicant listed for this patent is Korea Institute of Industrial Technology. Invention is credited to Sang Hun Heo, Jin Hun Jo, Ju Hee Kang, Ho Sung Kim, Hyo Sin Kim, Tae Won Kim, Yeong Mok Kim.
Application Number | 20150004526 14/377109 |
Document ID | / |
Family ID | 48867000 |
Filed Date | 2015-01-01 |
United States Patent
Application |
20150004526 |
Kind Code |
A1 |
Kim; Ho Sung ; et
al. |
January 1, 2015 |
TECHNIQUE FOR DESIGNING AND MANUFACTURING SOLID OXIDE FUEL CELL
HAVING IMPROVED OUTPUT CAPABILITY IN MID TO LOW TEMPERATURE
Abstract
The present invention relates to a technique for manufacturing a
unit cell for a solid oxide fuel cell (SOFC) which can improve the
output of the unit cell of the solid oxide fuel cell, without
occurring cost due to an additional process. The unit cell of the
solid oxide fuel cell, comprises: a fuel electrode support body; a
fuel electrode reaction layer; an electrolyte; and an air
electrode, wherein the fuel electrode support body is made from an
NiO and YSZ mixed material, the fuel electrode reaction layer is
made from a CeScSZ and NiO mixed material, the electrolyte is made
from a CeCsSZ material, and wherein the air electrode is made from
an LSM and CeScSZ mixed material.
Inventors: |
Kim; Ho Sung; (Suwon-si,
KR) ; Kang; Ju Hee; (Gwangju-si, KR) ; Kim;
Hyo Sin; (Seoul, KR) ; Jo; Jin Hun;
(Hwaseong-si, KR) ; Kim; Yeong Mok; (Daegu-si,
KR) ; Heo; Sang Hun; (Changwon-si, KR) ; Kim;
Tae Won; (Gwangju, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Korea Institute of Industrial Technology |
Cheonan-si |
|
KR |
|
|
Assignee: |
KOREA INSTITUTE OF INDUSTRIAL
TECHNOLOGY
Cheonan-si
KR
|
Family ID: |
48867000 |
Appl. No.: |
14/377109 |
Filed: |
November 20, 2012 |
PCT Filed: |
November 20, 2012 |
PCT NO: |
PCT/KR2012/009812 |
371 Date: |
August 6, 2014 |
Current U.S.
Class: |
429/489 ;
429/535 |
Current CPC
Class: |
H01M 4/8803 20130101;
H01M 2008/1293 20130101; H01M 4/9033 20130101; H01M 4/8889
20130101; H01M 4/8875 20130101; H01M 4/8885 20130101; H01M 8/1253
20130101; Y02P 70/56 20151101; Y02P 70/50 20151101; H01M 4/8835
20130101; H01M 2300/0077 20130101; H01M 8/1226 20130101; Y02E 60/50
20130101; H01M 8/126 20130101; H01M 4/9025 20130101; H01M 4/8857
20130101; Y02E 60/525 20130101; H01M 8/1213 20130101 |
Class at
Publication: |
429/489 ;
429/535 |
International
Class: |
H01M 8/12 20060101
H01M008/12; H01M 4/90 20060101 H01M004/90; H01M 4/88 20060101
H01M004/88 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 27, 2012 |
KR |
10 2012 0019682 |
Claims
1. A unit cell of a solid oxide fuel cell comprising: an anode
supporter formed of a mixture of Nickel(II) oxide (NiO) and
Yttria-stabilized zirconia (YSZ); an anode reaction layer formed of
a mixture of Cerium Scandia Stabilized Zirconia (CeScSZ) and NiO;
an electrolyte formed of CeScSZ; and a cathode formed of a mixture
of Lanthanum strontium cobalt (LSM) and CeScSZ.
2. The unit cell of claim 1, wherein the anode reaction layer, the
electrolyte, and the cathode comprise 1Ce10ScSZ powder.
3. The unit cell of claim 1, wherein the anode supporter, the anode
reaction layer and the electrolyte are manufactured by stacking and
cofiring a film manufactured by tape casting.
4. The unit cell of claim 3, wherein the cathode is manufactured by
screen printing.
5. The unit cell of claim 3, wherein the anode reaction layer is
manufactured by mixing a NiO powder and a CeScSZ powder at 46:54%
by weight (wt %).
6. The unit cell of claim 5, wherein the NiO powder has a size of
0.5 micrometers (.mu.m), and the CeScSZ powder has a size of 0.2 to
0.5 um and a specific surface area of 11 square meters/gram
(m.sup.2/g).
7. The unit cell of claim 3, wherein the electrolyte is
manufactured by mixing the CeScSZ powder and a solvent at 40:60 wt
%.
8. The unit cell of claim 7, wherein the CeScSZ powder has a size
of 0.2 to 0.5 .mu.m and a specific surface area of 11
m.sup.2/g.
9. The unit cell of claim 3, wherein the cathode is manufactured by
mixing LSM powder and CeScSZ powder at a weight ratio 1:1 (wt
%).
10. A method of manufacturing a unit cell of a solid oxide fuel
cell, the method comprising: preparing an anode supporter slurry by
mixing Nickel(II) oxide (NiO) powder and Yttria-stabilized zirconia
(YSZ) powder; preparing an anode reaction layer slurry by mixing
Cerium Scandia Stabilized Zirconia (CeScSZ) powder and NiO powder;
preparing an electrolyte slurry using CeScSZ powder; manufacturing
an anode supporter sheet using the anode supporter slurry by tape
casting; manufacturing an anode reaction layer sheet using the
anode reaction layer slurry by tape casting; manufacturing an
electrolyte sheet using the electrolyte slurry by tape casting;
manufacturing an anode supporter-electrolyte assembly by
sequentially stacking the anode supporter sheet, the anode reaction
layer sheet and the electrolyte sheet; manufacturing an anode
supporter-electrolyte by calcining and cofiring the anode
supporter-electrolyte assembly; preparing cathode paste by mixing
Lanthanum strontium cobalt (LSM) powder and CeScSZ powder; applying
the cathode paste to the anode supporter-electrolyte by screen
printing; and performing sintering.
11. The method of claim 10, wherein the anode reaction layer
slurry, the electrolyte slurry, and the cathode paste comprise
1Ce10ScSZ powder.
12. The method of claim 11, wherein the anode reaction layer slurry
is manufactured by mixing the NiO powder and the CeScSZ powder at
46:54% by weight (wt %).
13. The method of claim 12, wherein the NiO powder has a size of
0.5 .mu.m, and the CeScSZ powder has a size of 0.2 to 0.5
micrometers (.mu.m) and a specific surface area of 11 square
meters/gram (m.sup.2/g).
14. The method of claim 11, wherein the electrolyte slurry is
prepared by mixing the CeScSZ powder and a solvent at 40:60 wt
%.
15. The method of claim 14, wherein the CeScSZ, powder has a size
of 0.2 to 0.5 .mu.m and a specific surface area of 11
m.sup.2/g.
16. The method of claim 11, wherein the cathode paste is prepared
by mixing the LSM powder and the CeScSZ powder at a weight ratio
1:1 (wt %).
17. The method of claim 10, wherein the manufacturing of the anode
supporter-electrolyte assembly stacks the anode supporter sheet,
the anode reaction layer sheet and the electrolyte sheet
alternately in a first direction and a second direction
perpendicular to the first direction.
18. The method of claim 10, wherein the manufacturing of the anode
supporter-electrolyte mounts a flat alumina ceramic supporter with
a fixed size and weight on the anode supporter-electrolyte assembly
and conducts cofiring at 1,350.degree. C. while exerting a force of
about 40 kilograms/square centimeter (kg/cm.sup.2).
Description
TECHNICAL FIELD
[0001] The present invention relates to a method of manufacturing a
unit cell of a solid oxide fuel cell (SOFC), and more particularly
to material compositions of an electrolyte layer and a cathode, and
a process of designing and manufacturing a unit cell for achieving
high output of an SOFC unit cell.
BACKGROUND ART
[0002] A solid oxide fuel cell (SOFC) operates at a high
temperature of about 900 to 1000.degree. C., and thus, exhibits
superior electric power generating efficiency in comparison to
other fuel cells. However, deterioration of fine structures of an
anode, an electrolyte layer, and a cathode forming a unit cell
caused by operation at high temperatures, restrictions in
application of ceramic materials and an expensive manufacturing
process bring about durability, reliability, and economic
feasibility problems. As such, delays in practical utilization of
SOFCs are prominent. Accordingly, in recent years, research and
development is being conducted on reducing the operating
temperature of SOFCs to a medium-low temperature of about 700 to
800.degree. C. and in employing inexpensive metallic materials for
an interconnector instead of expensive ceramic materials. A
conventional SOFC unit cell operating at a high temperature of 900
to 1,000.degree. C. is formed of an anode supporter, an anode
reaction layer, an electrolyte and a cathode, wherein the anode
supporter includes Nickel(II) oxide-Yttria-stabilized zirconia
(NiO--YSZ), the anode reaction layer includes YSZ, and the cathode
includes a Lanthanum strontium manganite (LSM) material so as to
maintain mechanical properties of the ceramic unit cell.
[0003] Conventional SOFC unit cells are required to operate at a
high temperature of at least 800.degree. C. with the foregoing
structure. Since output characteristics of an SOFC increase in
proportion to an operating temperature, raising the operating
temperature is favorable for efficient generation of electricity,
whereas deterioration of the unit cell by a rise in temperature
introduces durability issues and increased costs. Specifically, in
an SOFC unit cell manufactured using a YSZ electrolyte material
among conventional SOFC materials, substantial changes in an ohmic
resistance occur according to the operating temperature.
Particularly, a drastically increase occurs at an operating
temperature of 800.degree. C. or lower, for example, about
700.degree. C., leading to a dramatic decrease in overall output
characteristics of the SOFC unit cell. That is, the SOFC unit cell
manufactured by conventional technology may exhibit low output
performance, for example, about 0.35 watts/square centimeter
(W/cm.sup.2) at an operating temperature of about 750.degree.
C.
[0004] Accordingly, there is a need to investigate design
technology, new materials, and manufacturing processes for an SOFC
unit cell which prevents a decrease in power output while
maintaining an SOFC operating temperature of 800.degree. C. or
lower. That is, to resolve a decrease in output performance due to
decreasing the SOFC operation temperature, use of Cerium (Ce) or
Scandia Stabilized Zirconia (ScSZ) based solid electrolyte
materials having excellent oxygen ion conductivity, instead of a
conventional YSZ solid electrolyte, is being actively studied for
reducing ohmic resistance by transferring oxygen ions in a unit
cell. Also, a Lanthanum strontium cobalt ferrite (LSCF) material
having excellent ion conductivity and electron conductivity is
being researched for a cathode, instead of a conventional LSM
material.
[0005] In the conventional technology, when the SOFC operating
temperature is reduced to a medium-low temperature, electrochemical
reaction properties are relatively deteriorated to increase the
ohmic resistance of an electrolyte and electrochemical polarization
resistance of a cathode, causing a considerable deterioration in
output characteristics of an SOFC unit cell. Accordingly, rigorous
studies are being conducted to derive an optimal output using the
conventional materials. That is, manufacture of an SOFC unit cell
which prevents a voltage drop due to a thinning electrolyte layer
of a conventional YSZ material, employs novel solid electrolyte
materials with excellent ion conductivity, such as Ce and ScSZ
materials, and uses an LSCF material with superior conductivity,
and catalytic performance for a cathode are being investigated.
[0006] There exists a design for an SOFC unit cell using ScSZ and
Gadolinia-doped ceria (GDC)-based solid electrolytes having
excellent ion conductivity and an LSCF material with superior
electron conductivity. That is, designing and manufacturing an SOFC
unit cell which uses conventional Ni--YSZ as an anode supporter,
NiO--CeScSZ or NiO--GDC materials as an anode reaction layer,
CeScSC or GDC materials as an electrolyte layer, and LSCF--CeScSZ
or LSCF--GDC materials as a cathode is being examined. However, an
LSCF cathode material reacts with a YSZ or ScSZ electrolyte to
cause a dual-phase reaction on an interface between the electrolyte
and the cathode, and thus, remarkably reducing output of the SOFC
unit cell. Thus, suppression of such a side reaction is necessary
to apply the LSCF material as a high-performance cathode material
to the unit cell, and accordingly an interface film of a thin-film
GDC electrolyte material is additionally needed between the cathode
and the electrolyte. However, since the GDC material has poor
sinterability and a higher sintering temperature than the ScSZ or
YSZ materials, the electrolyte layer provides debased fineness
after sintering acting as a new source ohmic resistance, thereby
insignificantly improving practical output performance or rather
reducing output characteristics according to circumstances and
increasing manufacturing costs. Thus, an optimal design and an
inexpensive manufacturing process for an SOFC unit cell based on
convergence of the conventional technology and new material
technology currently being developed is needed. Although
manufacture of high-output SOFC unit cells operating at a
medium-low temperature of about 750.degree. C. has been recently
researched and developed, an SOFC unit cell adopting a GDC buffer
layer for controlling reaction between the YSZ electrolyte material
and the LSCF cathode material involves a dual-phase reaction and
process control problems.
DISCLOSURE OF INVENTION
Technical Goals
[0007] The present invention is conceived to solve the
aforementioned issues, such as limits in maintaining mechanical
properties due to a thinning YSZ electrolyte layer, and thus, is to
provide a process of designing and manufacturing a solid oxide fuel
cell (SOFC) using materials with excellent electrical conductivity
for a solid electrolyte and a cathode.
Technical Solutions
[0008] According to an aspect of the present invention, there is
provided a unit cell of a solid oxide fuel cell (SOFC) including an
anode supporter formed of a mixture of Nickel(II) oxide (NiO) and
Yttria-stabilized zirconia (YSZ), an anode reaction layer formed of
a mixture of Cerium Scandia Stabilized Zirconia (CeScSZ) and NiO,
an electrolyte formed of CeScSZ, and a cathode formed of a mixture
of Lanthanum strontium cobalt (LSM) and CeScSZ.
[0009] The anode reaction layer, the electrolyte and the cathode
include 1Ce10ScSZ powder.
[0010] The anode supporter, the anode reaction layer and the
electrolyte are manufactured by stacking and cofiring a film
manufactured by tape casting. The cathode is manufactured by screen
printing. The anode reaction layer is manufactured by mixing a NiO
powder and a CeScSZ powder at 46:54% by weight (wt %). Here, the
NiO powder may have a size of 0.5 .mu.m, and the CeScSZ powder may
have a size of 0.2 to 0.5 micrometers (.mu.m) and a specific
surface area of 11 square meters/gram (m.sup.2/g). The electrolyte
is manufactured by mixing the CeScSZ powder and a solvent at 40:60
wt %. Here, the CeScSZ powder has a size of 0.2 to 0.5 .mu.m and a
specific surface area of 11 m.sup.2/g. The cathode is manufactured
by mixing a LSM powder and a CeScSZ powder at a weight ratio 1:1
(wt %).
[0011] According to another aspect of the present invention, there
is provided a method of manufacturing a unit cell of an SOFC, the
method including preparing an anode supporter slurry by mixing NiO
powder and YSZ powder, preparing an anode reaction layer slurry by
mixing CeScSZ powder and NiO powder, preparing an electrolyte
slurry using CeScSZ powder, manufacturing an anode supporter sheet
using the anode supporter slurry by tape casting, manufacturing an
anode reaction layer sheet using the anode reaction layer slurry by
tape casting, manufacturing an electrolyte sheet using the
electrolyte slurry by tape casting, manufacturing an anode
supporter-electrolyte assembly by sequentially stacking the anode
supporter sheet, the anode reaction layer sheet and the electrolyte
sheet, manufacturing an anode supporter-electrolyte by calcining
and cofiring the anode supporter-electrolyte assembly, preparing a
cathode paste by mixing the LSM powder and the CeScSZ powder,
applying the cathode paste to the anode supporter-electrolyte by
screen printing, and performing sintering.
[0012] The anode reaction layer slurry, the electrolyte slurry, and
the cathode paste include 1Ce10ScSZ powder. The anode reaction
layer slurry is manufactured by mixing the NiO powder and the
CeScSZ powder at 46:54 wt %. Here, the NiO powder has a size of 0.5
.mu.m, and the CeScSZ powder has a size of 0.2 to 0.5 .mu.m and a
specific surface area of 11 m.sup.2/g,. The electrolyte slurry is
prepared by mixing the CeScSZ powder and a solvent at 40:60 wt %.
Here, the CeScSZ powder has a size of 0.2 to 0.5 .mu.m and a
specific surface area of 11 m.sup.2/g. The cathode paste is
prepared by mixing the LSM powder and the CeScSZ powder at a weight
ratio 1:1 (wt %).
[0013] The manufacturing of the anode supporter-electrolyte
assembly stacks the anode supporter sheet, the anode reaction layer
sheet and the electrolyte sheet alternately in a first direction
and a second direction perpendicular to the first direction to
prevent warping and cracking. The manufacturing of the anode
supporter-electrolyte mounts a flat alumina ceramic supporter with
a fixed size and weight on the anode supporter-electrolyte assembly
and conducts cofiring at 1,350.degree. C. while exerting a force of
about 40 kilograms/square centimeter (kg/cm.sup.2).
Effects of the Invention
[0014] As described above, according to exemplary embodiments of
the present invention, CeScSZ as a solid electrolyte material with
high ion conductivity and a conventional LSM material as a cathode
material are used for a solid oxide fuel cell (SOFC), thereby
improving output a unit cell of the SOFC without involving cost due
to an additional process.
BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1 is a diagram illustrating a structure of a unit cell
of a solid oxide fuel cell (SOFC) according to an exemplary
embodiment of the present invention.
[0016] FIG. 2 is a graph illustrating output of the unit cell of
the SOFC according to the exemplary embodiment of the present
invention.
[0017] FIG. 3 is a graph illustrating impedance of the unit cell of
the SOFC according to the exemplary embodiment of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0018] While exemplary embodiments of the present invention will be
described below in detail with reference to the accompanying
drawings, the present invention is not limited to the exemplary
embodiments. In describing the present invention, detailed
descriptions of known functions or configurations may be omitted so
as to clarify the substance of the present invention.
[0019] Hereinafter, a process of designing and manufacturing a unit
cell of a solid oxide fuel cell (SOFC) according to an exemplary
embodiment of the present invention will be described in detail
with reference to FIGS. 1 to 3.
[0020] The unit cell of the SOFC according to the present
embodiment includes an anode supporter (NiO--YSZ), an anode
reaction layer (NiO--CeScSZ), an electrolyte (CeScSZ) and a cathode
(LSM). When lanthanum strontium manganite (LSM) is used for the
cathode, no additional processes are involved, output
characteristics of the unit cell are improved even when the SOFC
operates in a medium-low temperature range of 700 to 800.degree.
C., as in use of lanthanum strontium cobalt ferrite (LSCF), and an
inexpensive manufacturing process of the unit cell may be
maintained.
[0021] In detail, a process of manufacturing an SOFC unit cell that
designs an SOFC unit cell by combining an LSM material for a
cathode and a Scandia Stabilized Zirconia (ScSZ) based electrolyte
as a solid electrolyte with high ion conductivity, manufactures an
anode supporter-electrolyte assembly by tape casting and
manufactures a cathode layer by screen printing is developed,
thereby achieving high-output performance of the SOFC unit cell
even at medium-low temperatures without involving cost of an
additional manufacture process.
[0022] The anode reaction layer is manufactured by mixing
Nickel(II) oxide (NiO) and
[0023] Cerium Scandia Stabilized Zirconia (CeScSZ) materials at a
weight ratio of 50:50, forming, the mixture into a film by tape
casting and adjusting a thickness of the film to about 20 to 30
micrometers (.mu.m). To suppress polarization resistance of
hydrogen fuel, a CeScSZ material with a uniform particle size and a
uniform shape is used so that the supporter maintains a porosity of
about 40 to 60% even after hydrogen reduction of the NiO material.
A solid electrolyte layer is manufactured using a CeScSZ material
with excellent ion conductivity at a medium-low temperature of
750.degree. C. in the same manner as the anode reaction layer.
Then, an anode supporter film, an anode reaction layer film and an
electrolyte layer film are stacked and laminated, followed by
firing at about 1,350.degree. C. once, thereby simply manufacturing
the anode support-electrolyte assembly. A Lanthanum strontium
manganite (LSM) cathode slurry is applied directly to the
electrolyte layer, without interposing an additional buffer layer
film between the electrolyte layer of the anode support-electrolyte
assembly and the cathode, and is finally sintered at about
1,100.degree. C., thereby completing manufacture of an SOFC unit
cell.
EXAMPLE 1
[0024] The present invention employs 1 mol % Ce-doped 10 mol %
scandium-stabilized zirconia (1Ce10ScSZ) that has superior oxygen
ion conductivity in a medium-low temperature range to YSZ and a
similar coefficient of thermal expansion to that of an SOFC
electrode material and is relatively resistant to brittleness for
manufacturing an SOFC unit cell exhibiting high output capability
at a medium-low temperature. [0025] When 1Ce10ScSZ is used as a
material for an anode reaction layer, an electrolyte and a cathode
in manufacturing the SOFC unit cell, polarization resistance of the
SOFC unit cell is remarkably reduced when operating in a medium-low
temperature range of 750.degree. C. or lower. Thus, the unit cell
may exhibit an excellent output characteristic in a medium-low
temperature range of 750.degree. C. by using inexpensive LSM for
the cathode, in lieu of more expensive materials for medium-low
temperatures, such as Lanthanum strontium cobalt ferrite (LSCF) and
Gadolinia-doped ceria GDC.
[0026] The following example is provided for a full understanding
of the present invention in the art and may be modified or changed
variously without limiting the scope of the present invention to
the following example. A process of manufacturing the SOFC unit
cell is described in detail as follows.
[0027] First, an anode supporter was manufactured.
[0028] Anode supporter slurry was prepared using NiO and YSZ at
60:40, after which 30 to 40 anode sheets with a thickness of 40
.mu.m were stacked using the anode supporter slurry by tape
casting, thereby producing the anode supporter with a thickness of
about 1 to 1.2 millimeters (mm).
[0029] Next, an anode reaction layer slurry was prepared.
[0030] The anode reaction layer slurry was prepared by mixing about
0.5-.mu.m NiO powder and about 0.2 to 0.5-.mu.m CeScSZ powder with
a specific surface area of 11 square meters/gram (m.sup.2/g). Here,
the powder mixture and a solvent were mixed at 46:54% by weight (wt
%), and the NiO powder and the CeScSZ powder were mixed at 54:46 wt
% so that a ratio of Ni to CeScSZ was 4:6 after reduction. Toluene
and ethanol were used at a weight ratio of 20:13 as the solvent for
uniformly dispersing the powder mixture, and a dispersant (fish
oil) was added, followed by first ball milling at 200 revolutions
per minute (rpm) for 24 hours. After 24 hours, an equivalent amount
of a binder to a viscosity of 250 CentiPoises (cP) was added, after
which second ball milling was conducted for 24 hours, thereby
forming a reaction layer film with a thickness of 22 .mu.m by tape
casting.
[0031] Subsequently, an electrolyte slurry was prepared.
[0032] The electrolyte slurry was prepared using 1Ce10ScSZ powder
the same as used for the anode reaction layer slurry. The powder
and a solvent were mixed at a weight ratio of 40:60 wt %, in which
toluene and ethanol were used at 4:1 wt % as the solvent, and an
equivalent amount of a hinder to a viscosity of 700 cP and a
dispersant were added, thereby producing an electrolyte layer with
a thickness of about 10 .mu.m as a thin film in the same manner as
used for the anode reaction layer slurry.
[0033] Next, cathode paste was prepared.
[0034] The cathode paste was prepared by mixing a mixture of LSM
[(La.sub.0.7Sr.sub.0.3)MnO.sub.3-x] cathode powder and the same
1Ce10ScSZ powder with a solvent at a weight ratio of 70:30 wt %, in
which the LSM powder and CeScSZ powder were mixed at 1:1 wt %.
Here, ethyl cellulose and a-terphenol were used at 94:6 wt % as the
solvent. When a cathode is manufactured using the prepared cathode
paste by screen printing, the cathode may be of a good quality
including an excellent leveling effect without being affected by
surface defects of an electrolyte.
[0035] Subsequently, a unit cell was manufactured.
[0036] The 22-.mu.m cathode reaction layer (Ni/CeScSZ) was disposed
on the cathode supporter (Ni/YSZ/CB) with a thickness of about 1 to
1.2 mm, and the CeScSZ electrolyte as a 10-.mu.m thin film was
stacked on the cathode reaction layer to manufacture an anode
supporter-electrolyte assembly, after which the cathode
(LSM/CeScSZ) was applied to the electrolyte. Here, the anode
supporter, the anode reaction layer and the electrolyte were
manufactured by tape casting, while the cathode was manufactured by
screen printing.
[0037] In detail, an anode supporter slurry was prepared using Nb
and YSZ at 60:40, after which 30 to 40 anode sheets with a
thickness of 40 .mu.m were stacked using the anode supporter slurry
by tape casting, thereby producing the anode supporter with a
thickness of about 1 to 1.2 mm.
[0038] Particles in a film manufactured by tape casting are
arranged in a casting direction. Thus, when such films are stacked
in one direction, a unit cell may warp in a calcination process (of
removing organic matter and a pore forming agent). Thus, in the
present embodiment, the anode supporter, the anode reaction layer
and the electrolyte layer were stacked to cross in two different
directions. That is, one anode sheet was disposed in a first
direction, and another anode sheet was stacked thereon in a second
direction perpendicular to the first direction. In this way, the
anode sheets were sequentially stacked to cross in the first and
second directions, thereby forming the anode supporter.
[0039] Next, as described above, one sheet of the 22-.mu.m
Ni-CeScSZ anode reaction layer, obtained by mixing the NiO powder
and the CeScSZ powder, and the 10-.mu.m thin-film CeScSZ
electrolyte were sequentially stacked to form the anode
supporter-electrolyte assembly.
[0040] Next, the assembly was laminated by exerting constant force
at a constant temperature and subjected to calcination and
cofiring. Here, in lamination, a flat alumina ceramic supporter
with a fixed size and weight was mounted on the anode
supporter-electrolyte assembly, and a force of 450 kilograms/square
centimeter (kg/cm.sup.2) at 70.degree. C. was exerted for about 20
minutes.
[0041] In calcination, temperature was elevated to 1,000.degree.
C., maintained for 3 hours, and naturally cooled to room
temperature so as to remove the solvents, binders and carbon as a
pore forming agent included in the slurries for manufacturing the
anode supporter-electrolyte assembly.
[0042] Subsequently, a flat alumina ceramic supporter with a fixed
size and weight was mounted on the calcinated anode
supporter-electrolyte assembly and subjected to cofiring at
1,350.degree. C. while exerting a force of about 40 kg/cm.sup.2,
thereby manufacturing an anode supporter-electrolyte.
[0043] The cathode was applied to a thickness of about 40 .mu.m to
the electrolyte of the anode supporter-electrolyte by screen
printing and subjected to sintering, thereby producing the unit
cell.
[0044] The cathode was formed by applying the cathode paste four
times by screen printing into a multilayer structure with a
thickness of 40 .mu.m and sintering the structure at 1,100.degree.
C. for 3 hours.
[0045] The SOFC unit cell manufactured according to the present
invention was designed as in FIG. 1 and shaped as a coin cell to be
evaluated by a fuel cell evaluation system. The fuel cell
evaluation system evaluated I-V characteristics and impedance of
the coin cell-shaped unit cell as illustrated in FIG. 2. For
current-voltage analysis, at each operating temperature, H.sub.2
containing 3% water was flowed to the anode at 200
milliliters/minute (ml/min) and oxygen was flowed to the cathode at
300 ml/min. Current-voltage was measured by an electric loader
(model: PLZ664WA, KIKUSUI, Japan) using Pt mesh as a current
collector for the anode and the cathode.
[0046] Alternating current (AC) impedance of the cell was measured
in a frequency range from 100 kilohertz (kHz) to 0.02 hertz (Hz) at
an open circuit voltage and an amplitude of 10 millivolts (mV)
using an impedance analyzer (Frequency response analyzer, Solatron,
solatron 1260, U.S.A.).
[0047] Further, evaluation results of output characteristics of the
SOFC unit cell are illustrated in FIG. 2. As shown in FIG. 2, the
unit cell has an output of about 1.1 watts/square centimeter
(W/cm.sup.2) at a current density of 2.0 amperes/square centimeter
(A/cm.sup.2) when operating at a medium-low temperature of about
750.degree. C. Also, as a result of measuring impedance, the unit
cell exhibits an ohmic resistance of about 0.07 ohms-square
centimeter (.OMEGA.cm.sup.2) and a polarization resistance of 0.45
.OMEGA.cm.sup.2 at 750.degree. C., which are excellent output
characteristics as compared with a maximum output of about 0.23
W/cm.sup.2, an ohmic resistance of 0.4 .OMEGA.cm.sup.2 and a
polarization resistance of 0.6 .OMEGA.cm.sup.2 at the same
temperature.
[0048] The results show that the unit cell of the present invention
exhibits about two to three times better output characteristics
than when a conventional YSZ electrolyte is used and output
characteristics are about about 10% or more superior to an SOFC
unit cell employing a cathode of an LSCF material. Also, the unit
cell of the present invention involves a simplified manufacture
process and reduced cost as compared with a process using an LSCF
cathode material.
[0049] In the present invention, since a CeScSZ material is used
instead of a YSZ electrolyte, ohmic resistance of the unit cell is
considerably reduced. Also, the CeScSZ material does not cause a
dual-phase reaction with a cathode material and thus, the SOFC unit
cell may have substantially improved performance and exhibit high
output characteristics with reliability at a medium-low temperature
of 800.degree. C. or lower without involving additional cost due to
a manufacture process of the unit cell.
[0050] Although the present invention has been described with
reference to a few embodiments and the accompanying drawings, such
embodiments are provided for ease of understanding and the present
invention is not limited to the foregoing embodiments. Instead, it
would be appreciated by those skilled in the art that changes may
be made to these embodiments without departing from the principles
and spirit of the invention, the scope of which is defined by the
claims and their equivalents.
* * * * *